For certain telecommunications applications, in particular airborne applications, it is necessary to use flat antennas of very small thickness so as not to modify the aerodynamic profile of the carrier, for example when the antenna is positioned on the surface of an aircraft.
These telecommunication antennas comprise a plane surface comprising at least one radiating line able to transmit and receive signals of a frequency determined as a function of the shape of the radiating line. The signals are sent and received in the direction of the satellite which may be skewed with respect to the normal direction of the antenna as a function of the movements of the carrier. More specifically, these antennas must point a very directional beam inside a cone with a half-angle of at least 60° so that the antenna gain remains sufficient to guarantee the signal-to-noise ratio necessary for the quality of the link.
A known solution for carrying out this pointing consists in using a flat antenna 100 such as described in FIG. 1. This flat antenna 100 extends in a plane xy on an external wall 101 of an aircraft. Radiating lines 102 of the flat antenna 100 send and receive signals in a direction 103 skewed by an angle α with respect to the direction z normal to the surface of the flat antenna 100 in the plane perpendicular to the radiating lines 102 (xoz). This skewing requires an adjustment of the phase on each radiating line by means for example of programmable electronic phase shifters. The phase φi; to be displayed on line i so as to obtain a pointing in the direction α is given by the expression:φi=2πi d sin α/λ;with: i corresponding to the index of the line, d to the spacing between the lines and λ to the wavelength.
In order to skew the signals received in a cone, the flat antenna 100 is moreover movable in rotation β about an axis z orthonormal with the axes xy.
This first solution makes it possible to electronically scan all the pointing directions inside the cone.
However, the direction of the pointing in terms of α varies with the wavelength λ and does not allow simultaneous operation in two very different frequency bands such as in the Satcom Ka band for example (20 GHz when receiving, 30 GHz when sending).
To remedy this problem, it is known to use a ROTMAN lens described, for example, in U.S. Pat. No. 3,170,158. The ROTMAN lens is a known device making it possible customarily to obtain an antenna that radiates several beams that are skewed in a plane. The lens is furnished with N accessways each giving a beam in a frequency-independent given direction. Angular scanning is obtained by switching between the N available beams.
The lens is formed by the space between two parallel conducting planes, the input array consists of fixed horns embodied in waveguide form and radiating a polarization perpendicular to the metallic planes. The output array can consist of monopole type elements perpendicular to the metallic planes and making it possible to tap off the energy radiated by the horns of the input array. The linear array of radiating elements is fed by way of links (coaxial for example) whose lengths are such that the radiated wave is plane.
According to a similar principle, U.S. Pat. No. 8,284,102, discloses an electronic phase shifter comprising an electronic selector for a linear or curved array of sources. The focusing of the antenna is carried out by internal reflector elements and means of dielectric or refractive focusing.
This second solution makes it possible to have a fixed flat antenna on the surface of an aircraft. However, this solution limits the number of directions in which the antenna can be pointed as a function of the number of linear sources. Moreover, the installation of a linear array of sources and means of electronic selection increases the bulkiness of the flat antenna.
Furthermore, the ROTMAN lens is conventionally hooked up by coaxial cables connected between the ROTMAN lens and the radiating lines of the antenna. The length of the coaxial cables is adapted so as to introduce a delay required for focusing the wave radiated by the radiating lines for each horn of the ROTMAN lens. These cables are, of course, equipped with connectors at each end.
Such an antenna poses production problems when the antenna is designed to operate in the Ku or Ka high-frequency bands. Firstly, the length of the cables must be extremely precise so as to limit the errors in the phase. For example, for an antenna operating at 30 GHz, an error of 0.2 mm in the length of a coaxial cable induces a phase error of about 10°. Secondly, the size of the connectors of the coaxial cables limits the possibilities of installation and the number of usable horns. For example, for an antenna operating at 30 GHz, the spacing of the radiating lines and of the outputs of the Rotman lens is around 5 mm. Moreover, an antenna with a diameter of 500 mm operating at 30 GHz comprises about 100 cables, all different, this impacting negatively on the specifications and the steps of production.